Fans and blowers are used in many applications. For example, they have been used for blowing hot air away from power generators to cool down the generators. In some situations, the noise created by the fans or blowers can be very annoying to engineers working nearby. It is well known that long term exposure to noisy environment may have negative impact to people's hearing. Moreover, people tend to get tired more easily in noisy environment.
1. Past Approaches to Fan Noise Reduction
There are some approaches to fan noise reduction. Some of them require a redesign of the fans. Others have been proven to only work for computers. All of them may not be directly applicable to legacy fans or blowers in civilian and military systems. An ideal solution should be a low cost and portable active noise cancellation system that can be used in many diverse scenarios. The near field behavior of fan noise is complicated. However, at far field, the fan noise pattern is regular, which is similar to a spherical wave. The far field is defined as the square of the fan diameter divided by the sound wavelength. For a fan having a diameter of 1 ft., the distance to far field is about 1 ft. for a frequency of 1 kHz. One challenge is that the fan noise may consist of a band of frequencies, making it harder to suppress even at far field.
One prior active noise reduction system is disclosed in U.S. Pat. No. 9,117,457, issued on Aug. 25, 2015, by C. Kwan and J. Zhou, “Compact Plug-In Noise Cancellation Device,” which is useful for speech enhancement of cell phones and stethoscopes, but not as efficient when applied to fan noise reduction.
2. Proposed Active Noise Reduction Approach
The present invention proposes a novel and high-performance system to cancel fan or blower noise. The goal is to significantly reduce the noise at far field, which is more than 0.3 meter (1 ft.) for a fan size of 1 ft. in diameter and a noise frequency of 1 kHz. First, the present invention proposes to utilize 2 microphones: one to pick up the fan noise and the other one to pick up the noisy signal at far field. Second, the present invention proposes a portable loudspeaker that can be easily placed near the fan. The loudspeaker broadcasts omni-directional anti-phase signals to reduce the noise at far field. The present invention should perform well as the loudspeaker and the fan will look like point sources from the far field. Third, a real-time Digital Signal Processor (DSP) or Field Programmable Gate Array (FPGA) with fast adaptive filter is used to process the 2 microphone signals and generate the anti-phase signal. The adaptive filter uses the second microphone (fan noise) as a reference to generate an out-of-phase signal, which can then suppress the far field noise.
The key advantages of the present invention are briefly summarized as follows:                Simple setup and portable. The second microphone is placed in a small hardware box which contains the digital signal processor. This microphone should only pick up the fan noise. It should be placed close to the fan. The loudspeaker is compact and low cost (see FIG. 1). The loudspeaker should be placed very close to the fan so that both the loudspeaker and the fan will appear to be from the same point source from the far field. The whole system is portable.        High performance active noise suppression. The present invention is achieved by the fact that fan noise and the anti-phase signal from the loudspeaker look like spherical waves coming from the same point source far field. As a result, the two signals will cancel each other if the phase of the signal from the loudspeaker is adjusted appropriately.        Proven algorithms in noisy environments. The present invention utilizes proven adaptive algorithms to quickly compute the anti-phase signals.        
Details of the proposed system and software algorithm will be described below.